This invention relates to a method of preparing an improved creosote wood preservative from coal tar creosote. More particularly, this invention relates to a method of preparing an improved creosote wood preservative that has a more stable, and lighter color than coal tar creosote and that does not form a sticky crud deposit like conventional coal tar creosote.
Creosote has been used to preserve wood from decay and biological attack for many years. Creosote has been used to preserve, almost indefinitely, railroad crossties, wood blocks and timbers for buildings, tanks, bridges, telephone and telegraph and utility poles, crossarms, fence posts, and marine and foundation pilings and the like. Creosote consists principally of liquid and solid aromatic hydrocarbons. A more precise definition of creosote is that it consists of the 200.degree.-400.degree.C boiling fraction of coal tar distillate produced by the high temperature carbonization of bituminous coal. The main constituents of creosote have been classified by W. P. K. Findlay in "Preservation of Timber," Adams and Charles Black, London, 1962 as: (1) tar acids such as phenol, cresol, and xylenol, etc., and (2) tar bases such as pyridine, quinoline and acridine, and (3) neutral oils, such as a mixture of naphthalene, anthracene, and other neutral hydrocarbons.
Although creosote is an excellent wood preservative, it has several disadvantages. One disadvantage is that it has a dark color which is imparted to the treated wood. The color may become darker after exposure of the treated wood to air and light. A second disadvantage is that the creosote forms a sticky crud deposit on the treated wood after the treated wood is exposed to sunlight. Crud is a black sticky substance that can be defined chemically as the carbon disulfide insolubles of creosote after the creosote is exposed to ultraviolet radiation. After creosote is applied to a wooden pole, such as a utility pole, the pole is protected from decay and biological attack but the pole may have a blackish color and deposits of a black sticky substance, crud, often appear after exposure to sunlight.
The Cleveland Electric Illuminating Company has set color specifications, C.E.I. color specifications, for treated wood utility poles. These specifications set in the year 1965 and identified as Ser. No. 073 run from a value of 1 for a light colored creosote treated wood to a value of 9 for black colored treated wood pole. The other values between 1 and 9 are 3, 5, and 7 and they indicate an increased darkening color of the creosote treated wood. To meet the requirements of the utility company, the creosote treated wood must have a C.E.I. value of 5 or less. Wood specimens treated with a conventional creosote or a distillate of a conventional creosote have a C.E.I. value of 8.
One method used in the prior art to lighten the color of creosote is discussed in U.S. Pat. No. 3,046,217, W. F. Hefner et al. In this method the conventional creosote is heated to a temperature of about 100.degree. to 150.degree.C. While the creosote is maintained at this temperature, it is aerated for a period of about one hour. The easily oxidized compounds in the creosote are oxidized to stable oxidation products. Then, the air blown creosote is distilled to a temperature of about 395.degree. to 410.degree.C at atmospheric pressure. The purified light colored creosote is the distillate from the distillation step and the oxidized products remain behind in the residue. Next, the distillate is cooled to a temperature of about 40.degree.C to form crystals rich in anthracene, phenanthrene and carbazole. These crystals are removed from the distillate and the completed purified light colored creosote preservative is recovered.
There is a need in the art for a method of producing a light creosote wood preservative that imparts its light color to the treated wood and retains its light color after prolonged exposure to air and light. There is also a need in the art for a method of producing a light colored creosote wood preservative that does not form a sticky crud on the treated wood after the treated wood is exposed to sunlight.